1. Field of the Invention
The present invention relates to a moving picture encoding apparatus for performing a motion-compensated inter-frame predictive encoding and in particular, to a moving picture encoding apparatus having a camera controlling function that causes a camera to automatically track a moving object.
2. Description of the Related Art
Color information is used in one example of the automatic moving object tracking technology for keeping a moving object captured in the angle of a camera by moving the angle of the camera. In this example, the color information is extracted from a video signal in a designated detection block and registered as a specified color. The registered color is used for determining the motion of the object. In "Motion Vector Detection Circuit and Object Tracking Camera Apparatus Using the Circuit" by Matsumura et. al. disclosed in Japanese Patent Laid-Open Publication No. 8-125910, in order to prevent a color in a small area from being registered as a specified color of a block, only the color which is regarded as having high reliability on the basis of correlation is registered.
Japanese Patent Laid-Open Publication No. 4-126473 titled "Automatic Tracking Camera Apparatus" by Fujimoto et. al. discloses another example of the automatic moving object tracking technology. In this example, a contour of a moving object is extracted. Using the information in a closed space surrounded by the contour (color distribution information), a motion vector is detected by a known method. A camera is controlled to pan and tilt according to the motion vector. When the moving object goes out of the angle and become non-trackable, the width and height of the angle is controlled by zoom-out operations.
Still another example disclosed in "Global Zoom/Pan Estimation and Compensation for Video Compensation" (IEEE Conference of Acoustics, Speech and Signal Processing, 1991 M9, 4, pp. 2725-2728) is a method for decreasing the amount of generated information in a moving picture encoding device by globally compensating the motion of a camera using a global motion vector before performing conventional motion compensation. The global motion vector representing, for example, panning/tilting and zooming, is obtained by estimation based on local motion vectors for small blocks. However, the amount of calculation for obtaining the global motion vector from the local motion vectors is vast. Japanese Paten Laid-Open Publication No. 6-165164 titled "Dynamic Picture Processor" by Harasaki Hidenobu discloses a technique for decreasing the amount of the calculation by enlarging a motion vector search area and setting the area as a global area.
The aforementioned examples have the disadvantage that the quality of reproduced picture deteriorates to such a degree that a moving object becomes non-perceptible and the movement of the moving object becomes jerky while the camera is tracking the moving object because of the increase of generated information. Especially, the degree of the deterioration is intolerable when bit rate of transmission is low.
The reason of occurrence of the deterioration is explained as follows: When a camera automatically tracks a moving object, the background behind the moving object moves in the camera angle. Thus, the generated information increases. Then, in order to maintain a predetermined transmission rate, the moving picture encoding apparatus discards a part of generated information by coarsely performing the quantization, and decreasing the frame rate.
As explained above, when the camera tracks a moving object using a motion vector detected by a motion vector detecting means in a motion picture encoding apparatus, the background moves in the camera angle, the generated information increases, the motion picture encoding apparatus coarsely performs the quantization and decreases the frame rate in order to discard a part of the generated information, and the moving object become vague and the movement of the moving object becomes jerky.